It is well known that certain functionally substituted silanes, such as aminoalkyl or glycidoxy-alkyl silanes, when added to rubber compositions containing reinforcing grade mineral pigments, substantially increase the strength and resilience of the vulcanized rubber product. A preferred material for such purposes is 3-mercaptopropyl trimethoxysilane added in a small proportion to a vulcanizable rubber composition containing a reinforcing grade silica pigment. The function of the added material is believed to be the bonding of the pigment surface to the rubber matrix, by chemical linking of one end of the molecule to the pigment and the other end to the rubber.
Although terminal functional groups other than the mercapto group in a silane compound have been found to be effective in enhancing the strength and resilience of rubber containing silica or other mineral pigments, the best results are obtained with mercapto substituted compounds and particularly 3-mercaptopropyl trimethoxy silane. Unfortunately, all of these functionally substituted silanes are somewhat volatile, and tend to flash off and be lost during normal mixing in Banbury mixers, in which considerable heat is generated. This not only involves loss of an expensive material, but also creates problems because of the toxicity of the silanes.
In addition, the preferred 3-mercaptopropyl trimethoxy silane and other mercaptosilanes exhibit the characteristic property of mercaptans, of having a very objectionable foul odor, particularly at the relatively high temperatures at which rubber compositions are ordinarily mixed. The odor problem in the factories has seriously limited the use of these materials and the achievement of the highly desirable improvement in strength and resilience which they produce.